CosmicOS is a self-contained message designed to be understood primarily by treating it as a computer program and executing it. It is inspired by Hans Freudenthal's Lincos and resembles the programming language Scheme in many ways. Self-contained messages are of interest for CETI research, but there is much difference of opinion over the most appropriate encoding and broadcast medium to use. CosmicOS is released in modular form, so that the basic message can be adapted to a particular concrete instantiation. The message is released under the GPL licence.